The present invention relates to a color film image reader and, more particularly, to an apparatus adapted for reading an image of an orange-masked negative film.
The known systems for producing an image signal by reading an image from an exposed color film are roughly divided into a system using a black-and-white sensor and a system using a color sensor with a color filter chipped on a charge coupled device.
In the system using a black-and-white sensor, as shown in FIG. 9 for example, an exposed color film 10 is interposed between a light source 102 such as a tungsten lamp or the like and a color decomposing filter 103, and light transmitted through the film 101 is caused to be incident upon a solid state imaging device (CCD) 105, which is a black-and-white sensor, via a lens 104.
In the case of sampling a red component, the color decomposing filter 103 is so disposed that its red transmitting filter R is positioned between the color film 101 and the lens 104, whereby the light transmitted through the filter R is incident upon the CCD 105. The red light incident upon the CCD 105 is converted into an electric signal, which is then inputted via an amplifier 106 to a signal processing circuit 107 where a required signal process is executed.
A line sensor is employed for the CCD 105, which is scanned relative to the color film 101 so that a red signal of one image frame is obtained. Next the filter 103 is so rotated that a green transmitting filter G is positioned between the color film 101 and the lens 104, and an operation similar to the aforementioned is performed to thereby obtain a green signal of one image frame. Further the filter 103 is so rotated that a blue transmitting filter B is positioned between the color film 101 and the lens 104, and an operation similar to the aforementioned is performed to thereby obtain a blue signal of one image frame. The electric signals of the individual colors thus obtained are synthesized with one another to consequently reproduce the color image on the color film 101.
FIG. 10 shows an example of another image reader using a 3-band type fluorescent lamp 108 as a light source in place of a tungsten lamp. In the image reader of FIG. 10, the difference from the foregoing example is merely in the point that its light source is changed from the aforesaid tungsten lamp 102 to a 3-band type fluorescent lamp 108. Therefore a repeated explanation of the operation is omitted here. However, the spectral distribution of the tungsten lamp is such that the integral value of the spectral power in the area corresponding to R is in a range of 1/2 to 1/3 as compared with the integral value of the spectral power in the area corresponding to B as shown in FIG. 11; whereas in the case of using a 3-band type fluorescent lamp, the respective integral values of the spectral powers in the areas corresponding to R, G, B are approximately equal to one another as shown in FIG. 12, so that well-balanced R, G, B signals are outputted from the CCD 105.
According to the image reader employing a black-and-white sensor, it is necessary to execute scanning three times to obtain one image as described, whereby the reading time is prolonged with another disadvantage of a complicated structure. There is known another image reader with a black-and-white sensor wherein light sources are provided for R, G, B respectively and electric signals are successively obtained by switching the R, G, B light sources in succession. However, in the image reader of this type also, there exists the necessity of successively obtaining electric signals of individual colors, and some disadvantages are unavoidable including that the reading time is prolonged and the light sources are structurally complicated.
In a color negative film, a colored coupler known as an orange mask is employed to realize, in printing, clear and distinct coloring. Therefore a color negative film looks in orange, and its spectral density curve is such as graphically shown in FIG. 3.
In FIG. 3, the ordinate represents the spectral density D which is expressed as D=log 1/T (where T denotes the transmissivity of a color film). The minimum density shown in FIG. 3 is the spectral density in the orange mask, i.e., in the non-exposed area. Meanwhile the intermediate density shown therein is the spectral density obtained by an intermediate between non-exposure and exposure to white, i.e., by exposure to gray. As represented by the intermediate density in this spectral density curve, the spectral density of green (G) light is about 3.8 times that of red (R) light, and the spectral density of blue (B) light is about 10 times that of red (R) light. Namely, the transmissivity of blue (B) light through the color negative film is low, and the amount of the transmitted blue light is about 1/10. Meanwhile the amount of the green (G) light transmitted through the color negative film is about 1/4 as compared with that of the red (R) light. Accordingly, if electric signals of individual colors are directly synthesized with one another, it becomes impossible to reproduce the colors of the image on the color film. For this reason, there is executed a process of attenuating the levels of the green and red electric signals so that the level of the blue electric signal may be coincident with the levels of the green and red electric signals.
FIG. 11 graphically shows the spectral power distribution of a tungsten lamp. In FIG. 11, spectral powers in respective wavelengths are plotted with 100% corresponding to the spectral power in 555 nm where the visibility is maximum. As shown in FIG. 11, the spectral power distribution of a tungsten lamp is such that the spectral power of red (R) light ranging approximately from 400 to 500 nm, i.e., the integral value in this area, is about 1/2 to 1/3 of the spectral power of blue (B) light ranging approximately from 600 to 700 nm, i.e., the integral value in this area. Accordingly, in using a tungsten lamp as a light source, the correction mentioned above needs to be performed in consideration of the spectral powers as well.
However, when a 3-band type fluorescent lamp is used as a light source, the correction may be executed without giving any consideration to the spectral powers since the red, green and blue spectral powers are approximately equal to one another as shown in FIG. 12. The spectral power is determined by the integral value in the area of each color.
FIG. 1 shows an image reader employing a color sensor with a color filter chipped on a charge coupled device.
In the image reader of FIG. 1, an exposed color film 2 is positioned between a light source 1 such as a 3-band type fluorescent lamp and a lens 3, and the light transmitted through the color film 2 is caused to be incident, via the lens 3, upon a color charge coupled device (CCD) 4 which is a color sensor with red (R), green (G) and blue (B) color filters provided per pixel. Then, for example, a red component from the color film 2 is converted into an electric signal by pixels each furnished with a red transmitting filter. The red electric signal R thus converted is amplified by an RGB gain control circuit 5-1 and is further converted into a digital signal by an A-D converter 6-1 so as to be adapted for digital processing. The R digital signal thus obtained is then inputted to a matrix circuit 7.
Meanwhile a green component from the color film 2 is converted into an electric signal by pixels each furnished with a green transmitting filter. The green electric signal G thus converted is amplified by an RGB gain control circuit 5-2 and is further converted into a digital signal by an A-D converter 6-2 so as to be adapted for digital processing. The G digital signal thus obtained is then inputted to the matrix circuit 7.
Further a blue component from the color film 2 is converted into an electric signal by pixels each furnished with a blue transmitting filter. The blue electric signal B thus converted by the color CCD 4 is amplified by an RGB gain control circuit 5-3 and is further converted into a digital signal by an A-D converter 6-3 so as to be adapted for digital processing. The B digital signal thus obtained is then supplied, together with the R and G digital signals, to the matrix circuit 7 where color masking is executed, and the processed signals are stored in a frame memory 8. The color masking is a process of correcting the colors by mixing predetermined amounts of the R, G and B digital signals with one another.
The R, G and B digital signals stored in the frame memory 8 in this manner are inputted to D-A converters 9-1-9-3 respectively to be converted into analog signals, which are then delivered from R, G and B output terminals respectively.
As for the light source, a tungsten lamp may be used as well in place of a 3-band type fluorescent lamp.
In the image reader employing the color CCD 4, image data of one picture can be obtained by scanning merely once as described, so that it becomes possible to shorten the required reading time.
Since the color filter provided for the color CCD is structurally on-chip type, it is difficult to achieve sharp characteristics in the color filter. Therefore, the spectral sensitivity characteristics of the color CCD become such as graphically shown in FIG. 2 where, relative to the green (G) and blue (B) spectral sensitivity characteristics. The sensitivities thereof are raised with deviations from the green (G) and blue (B) areas on the long wavelength side.
Further, the spectral curve of a color negative film is such as shown in FIG. 3 where the intermediate spectral density of green (G) light is about 3.8 times that of red (R) light, and the intermediate spectral density of blue (B) light is about 10 times that of red (R) light. Namely, the transmissivity of blue (B) light through the color negative film is low, and the amount of the transmitted blue (B) light is about 1/10. Meanwhile the amount of the green (G) light transmitted through the color negative film is about 1/4 as compared with that of the red (R) light. Accordingly, if electric signals of individual colors are directly synthesized with one another, it becomes impossible to exactly reproduce the colors of the image on the color film. For this reason, there is executed a process of properly adjusting the gains of the RGB gain control circuits 5-1-5-3 which amplify the R, G and B electric signals respectively.
However, if the spectral density is corrected in the manner mentioned, it follows that the spectral sensitivity of the color CCD shown in FIG. 2 is also corrected. Consequently, the spectral characteristic of the color CCD after the correction is such as shown in FIG. 4. As is apparent from FIG. 4 where the sensitivity of green (G) light is indexed as 100%, the relative sensitivity of red (R) light is rendered extremely lower than that of blue (B) and green (G) light. In addition, since the characteristic curve is such that the sensitivity rises on the long wavelength side of green (G) and blue (B) light as mentioned, the green (G) and blue (B) components in the corresponding areas are detected as the red (R) component. As a result, there arises a problem that the colors are turned to be impure. Besides the above, in the use of a tungsten lamp, the spectral power of red (R) light ranges approximately from 1/2 to 1/3 in comparison with the spectral power of blue (B) light as shown in FIG. 11, so that an adequate correction needs to be executed in consideration of such spectral power difference. As a result, it becomes necessary to further lower the relative sensitivity of red light to eventually worsen the purity of colors.